1. Field of the Invention
The present invention relates to a system for positioning at high speed a transducer such as a magnetic head, an optical head and a print head etc. and more specifically to a positioning control system which has improved accuracy of positioning.
2. Description of the Prior Art
A storage apparatus such as magnetic disk apparatus and optical disk apparatus executes so-called head seek operation for moving the head to the target track position from the current track position on the disk by controlling an actuator mounted on the head. When the head is positioned to the designated target track position, data writing or reading operation is carried out through the head. Moreover, even in a recording apparatus such as a serial printer, X-Y plotter, etc., recording such as printing is carried out by moving and positioning the print head to the target position from the current position
FIG. 1 is a sectional view indicating a schematic structure of an ordinary magnetic disk apparatus of the prior art provided with a rotary actuator.
In FIG. 1, an enclosure 111 supports rotatably, for example, three sheets of magnetic disks 112 through a spindle 113 and these disks 112 are rotated at a constant speed, for example, of 3600 rpm with a spindle motor 114. Moreover, the magnetic-head 115 is attached to a head arm 117 through a support spring means 116 and is positioned to the designated track of the magnetic disk 112. The rotary actuator is composed of a rotary member 118 which fixes the head arm 117 and is rotatably supported by the enclosure 111 and a positioning motor for rotating the rotary member, for example, a voice coil motor 119, and rotates the magnetic head 115 for predetermined angle around the rotating axis of rotating member 118.
A magnetic disk apparatus for high density recording uses a closed loop servo control means for controlling such an actuator. This closed loop servo control means detects the current position of the magnetic head from the original position thereof by reading servo information on the magnetic disk with a magnetic head, also calculates distance to the designated track position from the current position, drives the positioning motor based on the calculated distance and positions the magnetic head on the designated track. FIG. 2 schematically shows an example of such a servo control system.
In FIG. 2, 115A denotes servo head for positioning; 115B, data read/write head; 121, rotary actuator; 122, 123, amplifier; 124, demodulator for demodulating the servo signal; 125, AD converter; 126, DA converter; 127, read/write control circuit; 128, motor control circuit; 129, main controller consisting of microprocessor. The same reference numerals are used for indicating the disk rotating system and head positioning system. This servo control system is formed by a closed loop of servo head 115A--amplifier 122--demodulator 124--AD converter 125--main controller 129--DA converter 126--amplifier 123--rotary actuator 121. The functions of these elements are already known and only the control of actuator in relation to the present invention will be explained here.
The main controller 129 comprises a memory to store tabulated data indicating a curve of target velocity corresponding to the moving distance of the head. A target velocity curve, which is calculated off-line is shown in FIG. 3, is used as a function of the number of tracks in the distance up to the target track position from the current track position. This target speed curve shows the deceleration characteristic for stopping the head at the target track position from a certain velocity thereof and the actuator is controlled corresponding to an error between the actual velocity of head and the target velocity curve. Therefore, since there is a large velocity error when the head seeking operation is started, when the voice coil motor of actuator is driven with maximum capability of the driving force and the actual velocity of the head coincides with the target velocity curve, the deceleration control is then carried out in accordance with the target velocity curve.
Control is generally realized with a structure introducing an analog circuit but the structure which realizes control with a digital circuit is also proposed.
The positioning control in the prior art realizes control of the head by outputting the target velocity curve, which indicates the deceleration characteristic and basically does not conduct control of acceleration. Accordingly, the high velocity seek operation requires a supply of heavy current to the voice coil motor of the actuator at the time of starting the seek and coincidence between actual velocity and target velocity curve within a short period of time, and also requires a large change in the drive current. Moreover, change of the drive current also becomes large when the acceleration mode is switched to the deceleration mode. Therefore, harmonics in the drive current increase during vibration due to resonance of the mechanical parts of the actuator including the magnetic head and decrease the accuracy of the head positioning. Therefore, it has been difficult to realize the high velocity seek operation.
For this condition, it is thought to control the head velocity for both acceleration and, deceleration of the seek operation but is difficult to realize this control because the analog circuit structure is complicated. Moreover, it can also be thought to realize this control with a digital circuit, but it is far from easy to realize control without derivation of vibration even when the head velocity is controlled for both acceleration and deceleration.
As a system for controlling velocity and acceleration of head in both acceleration and deceleration of a seek operation in order to prevent the problem explained above, namely vibration and noise of the actuator means in the seek operation of the head, two kinds of methods, U.S. Pat. No. 4,796,112 by M. Mizukami et. al. and U.S. Pat. No. 4,937,689 by Jay. S. Sunnyvale et. al. are proposed. These methods employ the trapezoidal wave as the acceleration and deceleration current (acceleration) in order to suppress vibration. Therefore, these methods are required to determine the shape of trapezoidal acceleration in accordance with each seek stroke. In other words, the time until the preset trapezoidal acceleration reaches the maximum value and minimum value and the time for switching the acceleration to deceleration must be set in detail. Particularly it is essential in the U.S. Pat. No. 4,796,112 to set the ratio of an upper side and a bottom side of the trapezoidal wave in accordance with the seek stroke. Accordingly, these methods have the disadvantage that the circuit structures or algorithms are very complicated for both analog and digital circuits.